The use of carbamoyl pyridinium compounds as cross-linking agents in photographic applications is known. See, for example, U.S. Pat. No. 4,063,952, which discloses carbamoyl pyridinium compounds having pyridine rings which carry a sulfo or sulfoalkyl group. According to the '952 method, the alkali metal salt of a starting pyridine compound is reacted with a carbamoyl halide in the presence of a solvent mixture comprising dimethylformamide and methanol, and the reaction product is actively precipitated by the addition of ethyl ether. The alkali metal halide by-product of this reaction precipitates with the desired carbamoyl pyridinium compound.
The carbamoyl pyridinium compounds of the '952 patent are described by the formula: ##STR1##
wherein:
R.sub.1 and R.sub.2 each represents an alkyl group, an aryl group, or an aralkyl group, or R.sub.1 and R.sub.2 together represent the groups required to complete a piperidine or a morpholine ring; PA1 R.sub.3 represents hydrogen, methyl or ethyl; PA1 R.sub.4 represents methylene, ethylene, propylene or a single chemical bond; PA1 Me.sup.+ represents an alkali metal cation such as Li.sup.+, Na.sup.+ or K.sup.+ ; and PA1 X.sup.- represents an anion such as Cl.sup.- or Br.sup.-. PA1 a) reacting a pyridine sulfonic acid compound with a carbamoyl halide in the presence of an effective amount of a by-product solubilizing solvent and an effective amount of a tertiary amine acid scavenger, thereby forming a reaction product comprising a carbamoyl pyridinium compound, a tertiary amine hydrohalide by-product and said by-product solubilizing solvent, wherein said amine hydrohalide by-product is substantially soluble in said by-product solubilizing solvent and said carbamoyl pyridinium compound is substantially insoluble in said by-product solubilizing solvent; and PA1 b) separating said carbamoyl pyridinium compound from said reaction product. PA1 R.sub.1 and R.sub.2 (which may be the same or different) each represents an alkyl group having: from 1 to 10 carbon atoms (e.g., methyl, ethyl, 2-ethylhexyl), an aryl group having from 6 to 15 carbon atoms (e.g., phenyl, naphthyl), aralkyl group having from 7 to 15 carbon atoms (e.g., benzyl, phenethyl), or R1 and R2 together form the atoms required to complete a heterocyclic ring (e.g., pyrrolidine, morpholine, piperidine, piperazine, 1,2,3,4-tetrahydroquinoline ring, etc.). X represents an halogen atom (e.g., Cl, Br). PA1 R.sub.3 represents a substituent such as hydrogen, a halogen atom, an alkyl group having from 1 to 10 carbon atoms (e.g., methyl, ethyl), an alkoxy group having from 1 to 10 carbon atoms, a carbamoyl group, or a ureido group. R4 represents an alkylene group having from 1 to 4 carbon atoms (e.g., methylene, ethylene, propylene) or a single chemical bond. PA1 R.sub.5, R.sub.6 and R.sub.7 (which may be the same or different) each represents an alkyl group having from 2 to 10 carbon atoms. (e.g., ethyl, propyl, butyl, 2-ethylhexyl, etc.), an aryl group having from 6 to 15 carbon atoms (e.g., phenyl, naphthyl), or a cycloalkyl group having from 5 to 8 ring carbon atoms (e.g., cyclohexyl). PA1 (A) is dimethyl carbamoyl chloride (DMCC), C.sub.3 H.sub.6 ClNO (MW 107.64); PA1 (B) is pyridine ethane sulfonic acid (PESA), C.sub.7 H.sub.9 NO.sub.3 S (MW 108.27); PA1 (C) is a suitable solvent, such as dichloroethane (DCE); PA1 (D) is a suitable tertiary amine, such as tributylamine (TBA), C.sub.12 H.sub.18 N (MW 185.36) PA1 (E) is DMC-PES, C.sub.10 H.sub.14 N.sub.2 O.sub.4 S (MW 258.35); and PA1 (F) is tributylamine hydrochloride.
J01-066,162 also describes the synthesis of carbamoyl pyridinium compounds having a pyridine ring which carries a sulfo or sulfoalkyl group. This process comprises forming the inner salt of a pyridine sulfonic acid in the presence of a tertiary amine and reacting the inner salt with a carbamoyl chloride. By this synthesis, carbamoyl pyridinium compounds free of inorganic salts, e.g., NaCl and KCl, are obtained.
DD 290,879 further describes the synthesis of carbamoyl pyridinium compounds having pyridine rings which carry a sulfo or sulfoalkyl group. This process, which involves reacting a pyridine sulfonic acid with a carbamoyl halide using a nitrile solvent and a tertiary amine, produces a tert-amine hydrochloride by-product. Acetonitrile, propionitrile, butyronitrile and benzonitrile are mentioned as suitable solvents. The object of this process is to provide a nitrile solvent in which the reactants and the tertiary amine hydrochloride by-product are soluble, and the carbamoyl pyridinium compounds are insoluble. It has been found, however, that carbamoyl pyridinium compounds prepared via this process still contain, after the reaction, significant amounts of by-product, which must be eliminated via subsequent washing steps with the nitrile solvent. The yield of the desired carbamoyl pyridinium compound is consequently lowered.
U.S. Pat. No. 5,559,239 discloses a process for preparing carbamoyl pyridinium compounds which involves reacting a pyridine sulfonic acid with a carbamoyl halide in the presence of triethylamine and a ketone (e.g. acetone) solvent. A mixture of the desired carbamoyl pyridinium compound and the tert-amine hydrochloride by-product is recovered from the reaction mixture and the carbamoyl pyridinium compound must then be separated from the by-product using a selective solvent, such as methanol. This second separation step increases the overall process cost and tends to lower yields. It is also believed that methanol washing may be detrimental to product quality.